Palestinians walk by the rubble of houses destroyed by Israeli strikes in Beit Hanoun, northern Gaza Strip, Saturday, July 26, 2014. Thousands of Gaza residents who had fled Israel-Hamas fighting streamed back to devastated border areas during a lull Saturday, and were met by large-scale destruction: scores of homes were pulverized, wreckage blocked roads and power cables dangled in the streets. (AP Photo/Lefteris Pitarakis)

Meanwhile, the Israeli army launched more than 40 attacks on the Gaza strip Sunday, killing 15 people, according to the Palestinian Health Ministry. More than 1,030 Palestinians have died in the course of the 20-day confrontation, while 46 Israelis have been killed.

International leaders had hoped that a temporary truce on Saturday could be extended through Sunday. U.N. Secretary Ban Ki-moon “urges, in the strongest terms, both the Israelis and Palestinians to extend, for an additional 24 hours, the humanitarian cease-fire that was in effect and mostly observed until early this morning,” his spokesperson said.

Israel and Hamas have placed blame on the other for the end to the fragile peace that had allowed many Palestinians to search their bombed-out homes for belongings and loved ones. Some 150 bodies were found in the rubble during the window in fighting, although hundreds more could still be buried, according to Palestinian health officials.

"Following Hamas' incessant rocket fire throughout the humanitarian window, which was agreed upon for the welfare of the civilian population in Gaza, the (military) will now resume its aerial, naval and ground activity in the Gaza Strip," Israel announced, adding that it had repeatedly urged residents of the Palestinian enclave not to approach "combat zones."

A column of smoke rose into the air above Gaza City, and NBC News teams in the area reported hearing incoming artillery shells, fighter jets and air strikes immediately after the announcement at around 10 a.m. (3 a.m. ET).

Clashes soon erupted between Israeli troops and Gaza militants, according to The Associated Press. The Islamic Jihad group said one of its field commanders was killed by tank fire near the southern Gaza town of Khan Younis, the AP added.

Israel says it is trying to prevent civilian casualties by sending evacuation warnings to residents in targeted areas, and blames Hamas for so many civilians being killed in the bombings. Palestinians counter that there are no safe havens in the strip. On Thursday, a U.N.-run school housing evacuees was bombed, killing 15.

Gazan authorities blamed the Israeli forces for the incident. The Israeli military said its troops were fighting Hamas in the area and that it was investigating. Netanyahu said Sunday that Hamas was firing from the school, but UNRWA Spokesman Chris Gunness countered that the U.N. is a humanitarian organization that does not take sides in the conflict.

Israel launched the operation on July 8 to stop rocket fire from the militant-ruled enclave, and to destroy tunnels that Hamas could use to infiltrate the country and get needed supplies, according to Israeli officials.

— Ayman Mohyeldin reported from Gaza, Yael Factor from Tel Aviv and F. Brinley Bruton from London. Reuters and The Associated Press contributed to this report.

First published July 27 2014, 12:16 AM

Ayman Mohyeldin

Since joining NBC News in September 2011, Mohyeldin has reported on the Arab world, including Egypt, Libya, Syria, Jordan, Turkey, Gaza and Lebanon. Inside Syria, Ayman traveled across the country reporting exclusively on the Syrian war, both with opposition rebels and government officials. He also has reported from Europe and across the U.S.

Prior to joining NBC, Mohyeldin was a correspondent for Al Jazeera English based in Cairo, where he was at the epicenter of Arab uprisings covering the revolutions in Tunisia and Egypt. From May 2008 until May 2010, Ayman was the only foreign broadcast journalist based in the Gaza Strip, a period in which he was the only American reporter covering the 2008-09 War on Gaza.

From 2003 to2006, he was based in Baghdad, covering the immediate aftermath of the U.S.-led invasion of Iraq, the daily struggle of ordinary Iraqis and the Iraqi insurgency. Mohyeldin was among the few international journalists allowed to observe and report on the U.S. handover of Saddam Hussein to an Iraqi judge.

In 2011, Time Magazine named Mohyeldin as one of the 100 Most Influential People in the World.

Mohyeldin's reporting has won a Peabody Award, the UK's Cutting Edge Media Award and Argentina's Perfil International Press Freedom Award. He also has received multiple Emmy nominations.

Mohyeldin was born in Cairo, Egypt and grew up in the U.S. and the Middle East.